1. Field of the Invention
The present invention relates to a compact lens system for an electronic still camera including a solid state imaging device such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS).
2. Description of the Related Art
As the use of electronic devices such as portable digital assistances (PDAs) and mobile devices has greatly increased, digital cameras and digital video units are frequently built in such electronic devices. Accordingly, demands for small sized cameras have increased. In order to satisfy these demands, a lens system that is smaller than that used in conventional digital cameras is required. A lens system included with mobile devices has more size limitations than a lens system mounted on a conventional digital camera.
On the other hand, an exit ray angle (hereinafter, referred to as an exit angle) of a principal ray can be realized within a range of 20° to 25° due to the development of solid state imaging devices, and accordingly, thin and small sized lens systems have been developed.
In order to fabricate a thin and small sized lens system, the pixel pitch of an imaging device should be smaller than 2 μm and the resolution of the imaging device should be improved. However, when the pixel pitch of the imaging device is reduced, the resolution of the imaging device is degraded, and thus, it is difficult to satisfy the above two conditions.
For example, a system having three lenses was suggested in order to realize a thin lens system; however, compensation of chromatic aberration with the pixel pitch of 2 μm or less cannot be satisfactorily performed, and thus, it is difficult to obtain the desired resolution.
In order to solve the above problem, a system having four lenses has been suggested. In this system, the resolution can be improved even when the pixel pitch is 2 μm or less by compensating the chromatic aberration; however, the overall length of the lens system is increased when compared to the diagonal size of the solid state imaging device. Thus, a thin lens system cannot be obtained.
For fabricating a thin lens system, the back focal length can be reduced; but if the back focal length is too short, the space for an infra red (IR) filter is reduced, and thus, a glass such as a cut filter cannot be inserted in the rear portion of the lens system or the adjusting space in an assembling process may be decreased. In addition, it is advantageous to realize a thin lens system when the exit angle is increased, but if a ray incident on the thin lens system is inclined, a shading problem may occur, that is, an actual aperture efficiency is reduced.